VO2 of eggs of the turtle Emydura macquarii, the crocodilian Alligator mississipiensis, and the tuatara Sphenodon punctatus, were measured throughout incubation. E. macquarii and A. mississipiensis, species in which hatching synchrony may be important, show a decline in VO2 prior to hatching ('peaked' pattern). This is similar to the pattern shown by ratite birds, where the decline period may be variable and facilitates hatching synchrony. The same interpretation is used here for reptiles. Hatching synchrony seems unimportant in S. punctatus, no decline in VO2 is observed, and the pattern of VO2 is similar to that shown by most precocial birds. Developmental asynchrony in reptilian nests, e.g., E. macquarii, probably results from temperature differences within the nest. Turtles with nests that are unlikely to experience developmental asynchrony, show truncation of the 'peaked' pattern, similar to the usual avian precocial pattern, or even extreme truncation approaching that of altricial birds. A pattern of ontogenetic VO2 in snakes (which have precocious young) similar to that in birds with altricial young may indicate a basic developmental difference in snakes.